The invention relates generally to gamma-ray logging in a borehole.
Measuring gamma-rays with a detector located within a borehole is a common operation in well logging.
Natural gamma-rays are emitted in a decay of subsurface materials such as thorium, uranium and potassium (Th, U, K), each of which emits a characteristic spectrum resulting from an emission of gamma-rays at various energies. The natural gamma-ray measurement is particularly useful in the exploration for exploitation of oil and gas resources because it is believed that the concentrations of Th, U, K taken individually or in combination are a good indication of previously unavailable information as to the presence, type and volume of shale or clay in the formations surrounding the borehole.
A detector in a spectral mode, i.e. a detector that is sensitive to the energy of the gamma-rays, may provide a gamma-ray spectrum, i.e. a gamma-ray count rate as a function of energy.
Furthermore, a gamma-ray detector may also detect neutron-induced gamma-rays. Using a neutron source in a logging tool for obtaining a characteristic of a formation surrounding a borehole is well known, particularly for measuring a formation porosity and lithology.
The neutron source may be an electronic generator of neutrons, which, using the d-T neutron reaction, allows to irradiate the formation with neutrons having a high energy (about 14 MeV). As a consequence, there is a significant number of nuclei in the formation, as well as in the borehole and inside the tool, which are transmuted into radioactive elements.
In particular, oxygen-16 nuclei may be converted into nitrogen-16 nuclei; the radioactive nitrogen atoms decay quickly by beta decay mostly to an excited state of oxygen, which in turn decays by emitting gamma-rays. A majority of the emitted gamma-rays have an energy around 6.1 MeV, which is much higher than gamma-ray energies from naturally occurring radioactive materials, which do not exceed 2.615 MeV. The drilling mud contains elements, in particular oxygen, that may be activated as explained above.
It is hence possible to detect within the borehole gamma-rays from a plurality of sources.
Gamma-ray logging may be performed during a drilling operation in the earth formation, so as to provide information about the formation surrounding a drilled portion of the borehole as soon as possible. FIG. 1 shows a schematic view of an example of a system for logging while drilling. A logging while drilling tool 108 comprises a drill bit 101 at an end of a drill string 103. The drill string 103 is used to drill a borehole 102. Logging tools (104, 105, 109) are disposed within the bottom hole assembly (BHA) 111, which is part of the drill string 103. Mud is carried through the mud channel 106 to the drill bit 101 at the bottom of the BHA 111. The drilling mud is pumped down from the surface to the drill bit 101, where it helps clear cuttings and bring them to the surface through an annulus between the drill string 103 and a formation 107.
One of the logging tools (104, 105, 114) may contain a neutron generator 110 that irradiates the formation 107 with high energy neutrons, so as to provide a mapping of the porosity of the formation 107 for instance. Such generator may be a pulsed neutron generator. A gamma-ray detector 109 may be provided close to the neutron generator to measure gamma-rays induced by the generated neutrons.
Furthermore, a gamma-ray detector 112 of the logging tool 105 may measure the natural gamma-ray activity of the formation 107. The gamma-ray detector 112 intended to measure the natural gamma-ray activity, may also detect gamma-rays produced by the decay of elements in the mud such as oxygen that were activated when they passed the neutron generator on the way to the bit. These additional gamma rays may perturb the natural gamma ray measurement. The gamma rays originating from the mud may be emitted when the mud passes in front of the gamma-ray detector inside of the assembly or in the annulus between the assembly and the formation. The signal obtained by the gamma-ray detector 112 may then need to be corrected to eliminate from the natural gamma-ray estimation the gamma-rays produced by the decay of radioactive elements created by the neutron flux of the generator.
The measurements taken downhole are transmitted to a downhole computer 113 comprising a memory storing the data obtained from the sensors and a processor for processing the obtained measurement. All or part of the processed information obtained from the measurements may be transmitted to a telemetry system (not shown) for communicating with the surface. Such telemetry system may be a mud pulse telemetry. Alternatively, the information may be retrieved from the computer memory when the BHA is brought back to the surface. The retrieved or transmitted data may be processed by a processor situated at the surface, at the wellsite or remotely.
An approach for correcting the gamma-ray estimation consists of determining one or more standard spectra for the gamma rays emitted by the activated mud. The one or more standard spectra may include one or more spectra related to a radioactive isotope (i.e. nitrogen-16) from the activation of oxygen by neutrons emitted by the neutron generator. The measured gamma-ray spectrum may then be analyzed as a combination of the standard spectra of the elements of the formation generating natural gamma-ray as well as the one or more standard spectra of the mud activation. From this analysis, it is possible to derive the part of the spectrum due to the mud activation and to correct the total count rate measured by the gamma ray detector by subtracting the count rate due to the detection of gamma rays from activated mud.
U.S. Pat. No. 7,081,616 from the Applicants already discloses a correction method including analyzing the measured gamma-ray spectrum, determining an interval count rate corresponding to the count of gamma-rays having an energy in a predetermined correction interval above a predetermined threshold (the threshold corresponding generally to an energy threshold that natural gamma rays do not reach) and determining a correction count rate from the interval count rate. The correction count rate, corresponding to the gamma rays derived from oxygen activation generated by the neutron generator may then subtracted from the total count rate.